MEMS Nanopositioner and Method of Fabrication

ABSTRACT

A microelectromechanical (MEMS) device is provided. The MEMS device comprises a substrate and a movable structure flexurally connected to the substrate, capable of moving in relation to the substrate, wherein the movable structure further comprising two or more segments having at least one mechanical connection between said segments to provide structural integrity of the moving structure; and wherein the at least one mechanical connection electrically isolates at least two segments

RELATED APPLICATIONS

This application claims priority to U.S. Provisional Application63/127,763, filed Dec. 18, 2020, the entirety of which is herebyincorporated by reference.

STATEMENT OF GOVERNMENT INTEREST

This invention was made with government support under Grant No.DE-EE0008322 awarded by the Department of Energy. The government hascertain rights in the invention.

BACKGROUND INFORMATION Field

This invention relates to a MEMS positioning device for positioning ascanning tip in scanning tunneling microscopy. It further relates tobatch fabrication of MEMS devices having electrical isolation betweencomponents.

Background

The Scanning Tunneling Microscope (STM) is one of the most powerful andversatile tools in nanotechnology. The STM is commonly employed toobtain topographic images from a conductive surface at atomic levels, aswell as to perform nanolithography, where a monolayer resist on asurface is patterned with atomic resolution. Thanks to its ultra-highresolution and atomic precision, the STM has found use in single atomadsorption/desorption and manipulation applications. All these featureshave contributed to the STM being recognized as a leading tool forfabrication of fascinating atomic-scale electronic devices. For example,STM may be used for fabrication of atomic wires, atomic-scale memories,atomic switches, atomic diodes, single molecule amplifiers, single atomtransistors, and solid-state quantum computers. Besides nanofabrication,the STM has been widely used for studying various characteristics ofmatter at the atomic scale.

The working principle of STM relies on the electron tunneling from ananometer-sharp tip to a conductive surface. An STM tip is usuallymounted directly on a three Degree-of-Freedom (DOF) piezotubenanopositioner to position the tip close to the sample surface, e.g.,about a nanometer or less. By applying an appropriate voltage biasbetween the tip and sample, electrons start tunnelling through thetip-sample gap. The tunneling current value is exponentiallyproportional to the tip-sample distance, thus the atomic topography ofthe sample causes the tunneling current to vary during a scan. Commonly,a control feedback loop is incorporated to maintain the tunnelingcurrent at a setpoint by keeping the tip-sample distance constant andregulating the Z-axis of the piezotube. STM images are constructed byplotting the controller output along the reference trajectories in theXY plane. The combination of the bias voltage, tunneling currentsetpoint, and electron dose values determines the STM mode for eitherimaging or lithography.

SUMMARY

An illustrative embodiment provides a microelectromechanical (MEMS)device comprising a substrate and a movable structure flexurallyconnected to the substrate, capable of moving in relation to thesubstrate, wherein the movable structure further comprising two or moresegments having at least one mechanical connection between said segmentsto provide structural integrity of the moving structure; and wherein theat least one mechanical connection electrically isolates at least twosegments.

Another illustrative embodiment provides a MEMS nanopositioner forscanning tunneling microscopy. The nanopositioner comprises: a substratehaving a flat surface defining a substrate plane, a first electrode, asecond electrode, and a set of substrate actuator fixtures; shuttle beammovable in a Z-direction, wherein the Z-direction is parallel to thesubstrate plane and aligned with a longitudinal axis of the shuttlebeam, the shuttle beam further comprising: a tip segment comprising aSTM tip electrically connected to the first electrode for sensingtunneling current; an actuation segment mechanically connected to thetip segment by a bridge segment; wherein the bridge segment provideselectrical isolation between the tip segment and the actuation segment;wherein the actuation and tip segments are flexibly connected to thesubstrate by a set of flexures; and wherein the actuation segmentfurther comprises a set of electrostatic actuator arms interposed withthe set of substrate actuator fixtures wherein the set of actuator armsare electrically connected to the second electrode for applying avoltage between the set of actuator arms and the set of substrateactuator fixtures.

The features and functions can be achieved independently in variousembodiments of the present disclosure or may be combined in yet otherembodiments in which further details can be seen with reference to thefollowing description and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the illustrativeembodiments are set forth in the appended claims. The illustrativeembodiments, however, as well as a preferred mode of use, furtherobjectives and features thereof, will best be understood by reference tothe following detailed description of an illustrative embodiment of thepresent disclosure when read in conjunction with the accompanyingdrawings, wherein:

FIG. 1A is a schematic diagram of a scanning tunneling microscope (STM)incorporating a MEMS nanopositioner device in accordance with anillustrative embodiment;

FIG. 1B is a schematic diagram of a MEMS nanopositioner device inaccordance with an illustrative embodiment;

FIG. 1C is a perspective view of a MEMS nanopositioner device inrelation to a substrate in accordance with an illustrative embodiment;

FIG. 2 is a schematic diagram of a silicon STM tip in accordance with anillustrative embodiment;

FIG. 3 is a schematic diagram of an oxide bridge in accordance with anillustrative embodiment;

FIG. 4 is a diagram of a simulated resonance mode shapes for a firstmode of a MEMS nanopositioner device in accordance with an illustrativeembodiment;

FIG. 5 is a set of graphs showing simulated displacement characteristicsof a MEMS nanopositioner device for amplitude response and phase angleresponse as a function of driving frequency in both air and ultra-highvacuum in accordance with an illustrative embodiment;

FIG. 6 is a set of graphs showing displacement amplitude response andphase angle response as a function of driving frequency of a MEMSnanopositioner device made in accordance with an illustrativeembodiment;

FIG. 7 is a flowchart of an overall fabrication process for a MEMSnanopositioner according to an illustrative embodiment;

FIG. 8 is a flowchart of a fabrication process for an electricallyisolating bridge segment of a MEMS nanopositioner according to anillustrative embodiment;

FIG. 9 is a flowchart of a fabrication process for a silicon STM tip fora MEMS nanopositioner according to an illustrative embodiment;

FIG. 10 is a set of schematic diagrams showing a microfabrication flowof a MEMS nanopositioner device in accordance with an illustrativeembodiment;

FIG. 11 is a set of SEM images of a MEMS nanopositioner device duringthe fabrication process in accordance with an illustrative embodiment;

FIG. 12 is a set of SEM images of a MEMS nanopositioner devicefabricated in accordance with an illustrative embodiment;

FIG. 13 is an image of an ultrahigh-vacuum chamber of a commercialscanning tunneling microscope configured with a MEMS nanopositionerdevice and a close up image of the MEMS nanopositioner device mountedonto the piezopositioner of the scanning tunneling microscope inaccordance with an illustrative embodiment;

FIG. 14 is a block diagram of a Z-axis feedback loop of a scanningtunneling microscope system in accordance with an illustrativeembodiment; and

FIG. 15 is an STM image of experimental results showing imaging with ascanning tunneling microscope configured with a MEMS nanopositionerdevice in accordance with an illustrative embodiment.

DETAILED DESCRIPTION

The illustrative embodiments relate to a novel integrated tip for use ina scanning tunneling microscope (STM) and method of fabrication for theintegrated tip. Slow Z-axis dynamics of STM systems is a keycontributing factor to the traditionally slow scan speed of thisinstrument. A great majority of STM systems use piezotubenanopositioners for scanning. The piezotube bulkiness along with themass of STM tip assembly restrict the overall Z-axis bandwidth of thesystem to about 1 kHz. This limited bandwidth slows down the STMresponse to the sample topography changes. Herein, we disclose amicrofabrication process to build a Microelectromechanical-System (MEMS)nanopositioner for Z-axis positioning in STM with a tenfold bandwidthimprovement, and with a similar range of motion as existing STM systems.

The MEMS nanopositioner device features an integrated nanometer-sharpin-plane Si tip, compatible with conventional batch fabricationprocesses. In addition, a novel electrical isolation technique isprovided to electrically isolate the tip from the rest of the MEMSnanopositioner device. This enables a separate routing for tunnelingcurrent signal, enabling, for example, parallel STM tips fabricated froma single substrate. The fabricated MEMS nanopositioner device achieves1.6 μm motion with its first in-plane resonance beyond 10 kHz. Thecapability of this MEMS nanopositioner to replace the Z-axis componentof STMs is demonstrated through obtaining STM images and conductingSTM-based lithography on a H-passivated Si(100)-2×1 sample underultrahigh-vacuum condition.

It is noted here that traditional piezoelectric actuators of STMs losemore than 50% of their actuation capability in cryogenic conditions. Thenovel MEMS nanopositioner devices described in the present disclosureare highly suitable for low-temperature STM as they maintain their fullrange of motion in cryogenic conditions.

Despite its critical role in a variety of applications, conventional STMsystems are not able to meet the requirements of the emergingapplications in nanotechnology. STM is a fundamentally slow system, andits scan speed and throughput are limited for several reasons, likelimited bandwidth of the piezotubes in the XY plane, underdampeddynamics of the piezotubes, and limited bandwidth of the currentsensors. Another important limiting factor is the bandwidth of thepiezotubes for Z-axis positioning, which determines how fast thepiezotube can respond to changes in the sample topography during a scan.Since the piezotubes are bulky components, their Z-axis bandwidth istypically limited to about 1 kHz with the tip assembly loaded.

To address the bandwidth limitation issue, researchers have proposedflexure-guided and dual-stage piezo nanopositioners. However, thesetechnologies are yet to find their way into scanning tunnelingmicroscopy. This is mainly due to their bulkiness and difficulties withtheir utilization in high-throughput nanofabrication applications. In anattempt to fix issues arising from bulkiness and high throughpututilization, flexure-guided nanopositioners have been miniaturizedthrough microfabrication processes. These devices offer large lateralbandwidths, a small footprint, and the potential for use in arrayconfigurations. Despite such benefits, most of these devices suffer fromslow Z-axis dynamics, ultimately limiting the scan speed.

Disclosed is a 1-DOF Microelectromechanical-Systems (MEMS)nanopositioner device to replace the Z-axis of conventional STMpiezo-nanopositioners. The design goal set for the MEMS nanopositionerdevice was to increase the Z-axis bandwidth of the STM system up to 10kHz, while maintaining a Range of Motion (ROM) of approximately 2 μm.First, we take advantage of anisotropic wet etching of doped Si torealize a sharp in-plane tip, which is suitable for batch production.This method avoids the need for a post-fabrication process to deposit aPt tip through Focus Ion Beam (FIB) deposition, which could be tedious,incompatible with batch fabrication processes, and requires skilledoversight. Moreover, we develop a novel electrical isolation techniqueusing standard cleanroom tools to realize a moving shuttle beamcomprised of segments that are electrically isolated, but mechanicallyconnected. This allows us to separate the electrical routing for the tipand shuttle actuation within the device layer of a Silicon-on-Insulator(SOI) wafer, in contrast to a double-SOI wafer approach demonstrated inprevious work.

A schematic overview of an STM system and a MEMS nanpositioner device inaccordance with an embodiment of the invention is illustrated in FIGS.1A and 1B, respectively.

Referring to FIG. 1A, a scanning tunneling microscope (STM) system 1comprises an adjustable mount for controlling XY position 7 to which apiezotube positioner 5 is held above a sample surface 8 (where thesample surface is in the XY plane shown). STM system 1 further comprisesa Z-controller 2 connected to the piezotube positioner 5 and a MEMSnanopositioner 100. A STM tip 105, extending from MEMS nanopositioner100 along the Z-axis, is positioned close to the sample surface at atip-sample height δ of about a nanometer or less. By applying anappropriate voltage bias 3 between the tip and sample, electrons tunnelthrough the tip-sample gap 7. The tunneling current is exponentiallyproportional to the tip-sample gap 8, thus the atomic topography of thesample causes the tunneling current to vary during a scan. The tunnelingcurrent is amplified by preamp 6 and conditioned by signal conditioningelectronics 4 to form an appropriate feedback signal in a controlfeedback loop. The control feedback loop maintains the tunneling currentat a setpoint by keeping the tip-sample gap constant. The tip-sample gapis controlled by regulating the MEMS nanopositioner. STM images areconstructed by plotting a controller output signal 9 along the referencetrajectories in the XY plane. According to an embodiment of the presentinvention, the controller output signal is the voltage applied to theMEMS nanopositioner to maintain the tunneling current at the levelspecified by the setpoint. The combination of the bias voltage,tunneling current setpoint, and electron dose values determines the STMmode for either imaging or lithography.

Referring to FIG. 1B, a schematic diagram of MEMS nanopositioner device100 is shown and in FIG. 1C, a perspective view of MEMS nanopositionerdevice 100 built on substrate 150 is shown. Device 100 includes astructure which is movable primarily in one direction with respect to afixed island 110. Fixed island 110 is an unreleased portion of thedevice layer of substrate 150 from which device 100 is fabricated.Device 100 comprises a tip segment 101 mechanically connected to ashuttle beam 102 which is movably suspended by a set of compliantflexures 120 attached to fixed island 110. An in-plane STM tip 105 isfabricated at the end of tip segment 101 from which the MEMSnanopositioner approaches the sample surface. An oxide bridge 103 isimplemented between tip segment 101 and shuttle beam 102, whichelectrically isolates tip segment 101 from the rest of the MEMSnanopositioner. MEMS nanopositioner device 100 further includeselectrostatic actuators 104 to accomplish movement of the shuttle beamand tip segment in the Z direction. Electrostatic actuators 104 comprisea set of actuator arms 104 a attached to the shuttle beam and anopposing set of fixed arms 104 b attached to the fixed island 110. Highstiffness of the flexures along Y direction, restricts the beam frommoving in the Y direction. The out-of-plane stiffness of the devicealong the X direction is designed to be higher than the in-planestiffness along the Z direction to reduce mechanical vibrations. Theactuators are also oriented towards Z direction.

According to FIGS. 1B and 1C, electrical signal routing of MEMSnanopositioner device 100 includes an electrical path 108 between STMtip 105 and tip electrode 141 through tip segment 101 (see also insetA). Electrical signal routing also includes electrical path 109 fromdevice electrode 142 to actuator arms 104 a via shuttle beam 102 (seeinset A of FIG. 4 wherein electrical path 109 routes over the topmostflexures attached to shuttle beam 102 to reach device electrode 142.Electrical path 109 is isolated from electrical path 108 via oxidebridge 103. Fixed island 110 is electrically isolated from electricalpaths 108 and 109 and connected to actuator electrode 143 via electricalpath 107. In some embodiments, the shuttle beam and the actuator arm 104a are held at ground potential (via device electrode 142), and avariable actuation voltage is applied to actuator arm 104 b and fixedisland 110 (via actuator electrode 143). Other major aspects of thedevice are described in detail in the following subsections.

We ultimately aim to integrate the device into a commercial STM byreplacing the STM tip with the MEMS nanopositioner device. Therefore,the MEMS nanopositioner device may be constructed to fit the area thatthe STM system permits with a range-of-motion similar to Z axis of atraditional STM piezotube (which is typically about 2 μm). In acommercial STM of interest to us, the tip is mounted on a tip holder,made up of a gold plate with outer diameter of 6 mm. This tip holderassembly is subsequently placed on a piezotube for tip positioning. Inaddition, the carrier on which the tip holder is transferred intoUltra-High-Vacuum (UHV) STM chamber, fits traditional STM tips. In someembodiments the traditional STM tips have less than 4.5 mm height. Tomeet such requirements, non-limiting embodiments are conceived and havebeen tested as described herein, wherein the MEMS nanopositioner deviceis constructed to be less than 6 mm wide in Y direction and less than4.5 mm long in Z direction with a range-of-motion of about 2 μm.

A conductive tip is a critical component of any STM setup, since itenables direct interaction with the surface and is responsible for theelectron tunneling. Therefore, the tip fabrication is of greatimportance given that the tip is required to successfully performimaging and lithography operations with the proposed device. Previously,a post-processing method of fabrication was used based on Focused IonBeam (FIB) deposition, to implement an STM tip on the microfabricateddevice. While sharp functional tips may be obtained with this method,its serial manner of fabrication substantially prolongs the devicefabrication process.

STM tip fabrication is integrated with a MEMS batch fabrication processfor the MEMS nanopositioner by taking advantage of anisotropic wetetching of highly doped Si. FIG. 2 shows schematics of the in-plane STMtip 205. A Silicon-on-Insulator (SOI) wafer with crystallographic <100>orientation is utilized to fabricate this tip. In this scheme,intersection of three planes forms a tip 206 at point A: plane ABC isformed by wet etching of Si, plane ABD is formed by vertical dry etchingof Si, and plane ACD is the bottom side of the SOI wafer's device layer.Since the tip is made of Si, its conductivity depends on the dopinglevel of the wafer. The geometry of the tip can be adjusted by twoparameters: t (210) and θ. The parameter t is the device layerthickness, and θ is the angle at which the tip body deviates from the Sicrystallographic <110> direction 215, defined by photolithographyprocess. The tip apex length, L 220, can be calculated as:

$\begin{matrix}{L = \frac{t}{\sqrt{2}\tan\theta}} & (1)\end{matrix}$

The angle of tip on the three aforementioned planes solely depends on θ:arctan (√3 tan θ), arctan (√2 sin θ), and θ, respectively. The lengthand angles are important characteristics of this tip, but their effectson the tip geometry are opposing. As the tip angles become narrower,which is more favorable for scanning tunneling microscopy, the tipbecomes longer. Longer tips are more susceptible to vibrations,deteriorating the STM performance. Therefore, a trade-off is made whileselecting θ and t. In this work, we consider θ=15 degrees and t=19 μm.The disclosed tip fabrication method makes the overall microfabricationprocess conducive to mass production and allows us to obtain tips withan average radius less than 10 nm throughout the wafer.

In the MEMS implementation of the Z-axis STM nanopositioner, both thetip and the electrostatic actuators are parts of the shuttle beam. Insome embodiments, an array configuration of MEMS nanopositioners andintegrated STM tips are conceived. In order to enable the use of thisdevice in an array configuration, it is necessary to isolate theelectrical routings of the tip from the shuttle. To realize this, in aprevious design, the top and bottom device layers of a double-SOI waferwere utilized to route out the tunneling current from the tip separatefrom the ground line going to the shuttle beam. Herein, the in-plane tipscheme requires the electrical isolation to be implemented in the samedevice layer as the tip and shuttle beam. Therefore, the tip and theshuttle beam are electrically isolated while remaining mechanicallyattached. To do this, a novel method is disclosed to fabricate acomposite shuttle beam consisting of two Si bodies connected to eachother by means of an oxide bridge. The isolation method relies on theconsumption of Si during wet thermal oxidation, and replacing theconductive slender Si beams bridging the shuttle sections with aninsulator, SiO2.

Referring now to FIG. 3, details of an oxide bridge are shown. Shuttlebeam 302 is mechanically connected to tip segment 301 via oxide bridge303. Oxide bridge 303 comprises a set of oxide beams 332 in the middle,a pair of oxide flexures 334 on either side with additional oxidepatches 333 on top of the shuttle beam 302 and tip segment 301 formechanical reinforcement of the oxide/Si interfaces 331. An embodimentof a bridge fabrication process is described below in relation to FIG.8.

In general, implementation of this scheme on the shuttle beam requiresoptimizing the mechanical integrity of the beams to prevent the failureof the oxide bridge under force. Since the oxide is brittle material,flexures are put at both sides of the bridge structure, and actuatorsare implemented unidirectionally to ensure that the bridge region willbe under compressive stress all the time. In addition, a preloadcompressing stress applied on the bridge when the tip is on the samplesurface ensures that the inertia of the tip will not cause tensilestress in the bridge region during high-speed applications. The authorsbelieve this is the first demonstration of a movable shuttle beamcomposed of two electrically separated sections capable of withstandingload and structural integrity.

Electrostatic parallel-plate actuation methodology is implemented inthis device, as this transduction mechanism offers high resolution, fastresponse, and low creep over a few micrometers of range. Theelectrostatic force and input voltage (neglecting the fringing fieldeffect) are related according to:

$\begin{matrix}{{F_{es} = {\frac{\varepsilon AV^{2}}{2\left( {d_{0} - z} \right)^{2}} = {{- k_{z}}z}}},} & (2)\end{matrix}$

where F_(es), ε, A, V, d₀, k_(z), and z are the electrostatic force,permittivity of the medium, total overlapping plate area, input voltageapplied to the plates, initial gap, stiffness in Z direction, anddisplacement, respectively. Of consideration is the effect of pull-ininstability, which occurs when z is larger than one-third of d₀. Toprotect the device, the maximum value of z (stroke of the device) and d₀are limited to 2 mm and 7 mm, respectively. As a secondary precaution, amechanical stopper is also implemented at the back-end of the shuttlebeam to mechanically restrict large displacements.

The dynamic mode of a MEMS nanopositioner can be modeled as a secondorder spring-mass-damper system, where the cumulative stiffness of thesuspension elements has a critical role both in device performance andfunctionality. Stiffness of the device along the Z-axis has to besufficiently high to achieve bandwidth greater than 10 kHz, and to beless susceptible to thermal noise and to avoid the snap-in effectarising from the intermolecular forces between the tip and sample duringa scan. In addition, the device needs to be compliant enough to reachthe desired stroke (i.e. 2 μm) with a reasonable level of actuationvoltage (i.e. less than 100V in a STM system).

Considering the aforementioned design criteria, a conservative estimatefor the minimum required stiffness along the Z-axis was calculated as112 Nm⁻¹ in previous work. Furthermore, thermal noise is a limitingfactor for high-precision scanning probe microscopy, since it can excitemechanical resonances of the device. Due to the fact that MEMS devicesare typically lightly damped, thermally excited resonances candeteriorate a system's performance. Based on the equipartition theorem,the thermal noise effect can be characterized by the following equationfor a typical oscillator:

$\begin{matrix}{{{\overset{\_}{x}}_{th} = \sqrt{\frac{k_{b}T}{k}}},} & (3)\end{matrix}$

Where x _(th) is the mean displacement induced by the thermal noise,k_(b) is the Boltzmann constant, T is temperature, and k is thestiffness along the desired direction. According to this equation,increasing the stiffness reduces thermally-induced vibrations. However,increased stiffness in Z direction leads to the requirement for a largeractuation force. Consequently, larger actuators become more prone todeflection, increasing the chance of electrical shorting. To address therequisite compromise, geometry of the double-clamped beam-typesuspension elements are optimized through the Finite Element Analysis(FEA) simulations. In these simulations, the in-plane stiffness valuealong Z direction is designed to be as high as 285 Nm⁻¹, while theout-of-plane stiffness of the device along the X direction is obtainedas 505 Nm⁻¹. For illustrative purposes, the stiffness of the flexures isestimated to be 622 kN/m along the Y axis. In other embodiments, thestiffnesses may vary somewhat from the example presented, but wouldtypically be of a similar order.

The MEMS nanopositioner device is designed with CoventorWare software.Based on the design parameters discussed in the previous subsections, webuilt a Computer Aided Design (CAD) model of the device shown in theperspective view of FIG. 1C. The overall dimensions of the device are4.3 mm in length and 6 mm in width. The nanopositioning device is 19μm-thick and comprises a 2.7 mm long shuttle beam with an in-plane tip,16 slender flexures, and 44 pairs of parallel-plate electrostaticactuators. Table 1 summarizes the dimensions of the device components.

TABLE 1 Length Width Small base Large base Feature type (μm) (μm) (μm)(μm) Flexures 750 12 — — Flexures with routing 750 16 — — Actuator arm730 — 10 30 Shuttle beam 2753 40 — — Tip 153 — — —

Since the nanopositioner is a distributed parameter system, its dynamicresponse is described by its natural frequencies and mode shapes. Hence,the modes shapes of the device are obtained using the FEA package, andshown in FIG. 4, which are diagrams of simulated resonance mode shapesfor (4(a)) first mode, (4(b)) second mode, (4(c)) third mode and (4(d))forth mode of a MEMS nanopositioner device wherein the correspondingmode resonances are at 13.7 kHz, 16.7 kHz, 18.8 kHz and 20.3 kHz,respectively. This demonstrates that the first natural frequency of thedevice is 13.7 kHz along the Z-axis, satisfying a key design criterion,while the remaining out-of-plane modes lie beyond 16 kHz.

In parallel-plate electrostatic actuators, the pull-in phenomena limitsthe maximum allowable displacement of the device. For an idealparallel-plate electrostatic actuator, the pull-in instability occurs atone-third of the initial gap. However, fringing field effect originatingfrom the limited geometry of the actuators contributes to thisinstability. In order to have an accurate estimation of the pull-involtage, the static response of the device under electrostatic actuationis simulated and illustrated in plot 501 of FIG. 5. The result predictsthat the shuttle beam displaces 2.2 μm in Z direction just before thepull-in instability occurs at 69.4V. Finally, because the Z-axis of thedevice is equipped with actuators, the resonance of the system alongthis axis needs to be obtained under electrostatic actuation force. Thesuperposition of a DC voltage, i.e. 20V, and a small-amplitude AC signalis applied to the device actuators in various frequencies. Underassumption of squeezed film damping between actuator plates, thedisplacement response of the shuttle in Z direction is determined foreach frequency and depicted in FIG. 5 which shows a graph 502, of theamplitude frequency response in air, a graph 503, of the amplitudefrequency response in ultra high vacuum (UHV) conditions. Also shown inFIG. 5, is a graph 504, of the phase frequency response in air and agraph 505 of the phase frequency response in UHV conditions. Simulationsshow that resonance happens at 13.6 kHz with 40.2 dB dynamic range,which is typical of lightly damped behavior of MEMS devices. Theresonance frequency obtained here is lower than the device's firstnatural frequency. This could be attributed to two causes: the softeningeffect of the applied DC voltage, and velocity damping effect. Since themajority of the STM systems work in UHV conditions, it is necessary toassess the UHV effect on the underdamped behavior of the MEMSnanopositioner device. We repeated the same simulation under UHVconditions with 10⁻¹⁰ Torr pressure, and included the result in FIG. 5.This simulation predicts that the dynamic range of the device increasesby 11.8 dB in UHV conditions

FIG. 6 shows measured displacement and frequency responsecharacteristics corresponding to the simulations of FIG. 5.

FIGS. 7-9 show details of a microfabrication process for the MEMSnanopositioner device. FIG. 10 illustrates the fabrication process in aset of schematics while FIG. 11 provides SEM images of a representativedevice during the fabrication process.

FIGS. 7, 8 and 9 provide flow diagrams of the overall fabricationprocess 700, bridge fabrication process 800 and STM tip fabricationprocess 900, respectively. FIG. 8 should be considered a non-limitingembodiment of a bridge fabrication process for step 702 of process 700.FIG. 9 should be considered a non-limiting embodiment of an STM tipfabrication for step 704 of process 700.

FIG. 10 shows (a) initial SOI wafer, (b) bridge body formation, (c)thermal oxidation with subsequent LPCVD oxide deposition, (d) RIE of theoxide, (e) DRIE of the device layer, (f) sacrificial oxide layerdeposition, (g) oxide opening formation with RIE, (h) wet etching of Si,(i) stripping the sacrificial oxide layer, (j) electrode deposition, (k)DRIE of the handle layer, and (l) releasing the device by RIE of the BOXlayer.

FIG. 11 shows SEM images of (a) beams (b) oxidizing the beams (c)exposing the Si device layer (d) DRIE of the device layer (e) tip body(f) sacrificial LPCVD oxide defined as a wet etch mask (g) in-plane tip(h) stripping the sacrificial oxide layer. All scale bars are 40 μm inthese images.

A highly doped SOI wafer (0.001-0.005 Ωcm, N-type, <100>) with a 20-μmdevice layer, 2-μm Buried-Oxide (BOX) layer, and 400-μm handle layer ischosen for the fabrication of the 1-DOF MEMS nanopositioner (step 701,illustrated in FIG. 10(a)). At step 702, the bridge fabrication processstarts with defining the electrically isolating bridge geometry.

The bridge fabrication process 800, shown in the flow chart of FIG. 8,begins at step 801, wherein a set of beams are etched in the devicelayer by etching the device layer down to the BOX layer with the DeepReactive Ion Etching (DRIE) process (See FIG. 10(b) and FIG. 11(a)).Typically, the beams are about 2 μm wide and about 20 μm long. Then, atstep 802, the wafer undergoes a wet thermal oxidation step at 1100° C.to thoroughly oxidize the bridge (FIG. 11(b)). Step 802 consumes thewhole of Si within the bridge structure and typically increases the beamwidth to about 4.4 μm. In order to reinforce the oxide bridge, a 0.8-μmoxide layer is conformally deposited all over the wafer, at step 803,using a low-pressure chemical vapor deposition (LPCVD) process (FIG.10(c)) increasing the beam width to typically about 6 μm. Two patches ofoxide (oxide patches 333 of FIG. 3) are intentionally left at eitherside of the oxide bridge to further strengthen oxide/Si interfaces 331(FIG. 3). The oxide bridge fabrication is concluded, at step 804, byetching back the oxide on the device layer by Reactive Ion Etching (RIE)process to expose the Si layer again for the subsequent steps (FIG.10(d) and FIG. 11(c)). In one embodiment, the initial Si layer thicknessis selected to be 20 μm, reducing to 19 μm after completion of the oxidebridge fabrication.

Returning to the overall fabrication process 700, all other componentsof the MEMS nanopositioner, such as shuttle beam, actuators, flexures,and bonding pads (electrodes) are formed at the same time by patterningand etching the device layer with DRIE process (FIG. 10(e) and FIG.11(d), step 703). At step 704, the STM tip fabrication process beginswherein the body of the STM tip is formed with the angle of θ withrespect to the Si crystallographic <110> plane (FIG. 11(e)). In thenon-limiting embodiments presented, the angle θ is selected to be 15°.In other embodiments, the angle θ may be chosen to be greater or less,between 6°-45°.

STM tip fabrication process 900 is further described in relation to FIG.9. After the STM tip body is formed, step 901 is performed wherein asacrificial 300-nm oxide layer is deposited with LPCVD process toconformally cover the STM tip body (FIG. 10(f)). Afterwards, at step902, a window in the oxide layer is opened on top of the tip body withRIE process to define a mask for Si wet etch (FIG. 10(g) and FIG.11(f)). Thanks to the sacrificial oxide layer, only the STM tip body isthen anisotropically etched away, at step 903, in a Si etch solution(e.g., 45% KOH solution or a TMAH solution) to form the STM tip (FIG.10(h) and FIG. 11(g)). The STM tip formation is concluded by etchingaway the sacrificial oxide layer (FIG. 10(i) and FIG. 11(h), step 904)with an oxide etch solution, for example, a buffered oxide etch solutionor concentrated HF.

Returning to FIG. 7, the overall fabrication process 700 continues atstep 705. In order to provide electrical signal routing and bondingpads, a stack of 20-nm Cr and 280-nm Au layers are deposited and thefeatures patterned on the device with a lift-off process or dry etchprocess (FIG. 10(j), step 705). In the present embodiment, a sputteringprocess was used to deposit the metal layer and a lift-off process wasused to pattern the bonding pads. In other embodiments, the metal layerdeposition is not limited to Au and the process for deposition may bechosen from other methods known in the art such as evaporation.

At the last steps of the fabrication, the handle layer is patterned andetched up to the BOX layer from the back side (FIG. 10(k), step 706). Inthe present embodiment, a DRIE process is used to remove the handlelayer back to the BOX layer. In other embodiments, other processes maybe used to remove the handle layer, for example, Si wet chemicaletching. Then, at step 707, the BOX layer is etched away and thenanopositioner is released (FIG. 10(l)). In the present embodiment, aRIE process is used to etch away the BOX layer. Other processes, such asa vapor HF etch, may be used in other embodiments to etch the BOX layer.FIG. 12 shows the Scanning Electron Microscopy (SEM) images of afabricated MEMS nanopositioner 1200, showing the STM tip at increasinglevels of magnification in images 1201, 1202 and 1203. Fabricated MEMSnanopositioner includes an image of oxide bridge 1204, an image of theactuator arms 1205 and an image of compliant flexures 1206.

Important characteristics of the fabricated MEMS nanopositioner areempirically measured here in order to extract necessary parameters forthe experiments. These properties include maximum achievabledisplacement, transfer function estimate for the first mode dynamics,and mode shapes of the device.

The actuation voltage-displacement trend of the device is experimentallyobserved to determine the required voltage levels for the desiredstroke. During the experiments, a 4 Hz triangular signal is applied tothe electrostatic actuators using a function generator cascaded with avoltage amplifier. Then, the displacement trend for the correspondingactuation voltage is simultaneously monitored using a Polytec MSA-100-3DLaser Doppler Vibrometer (LDV). The measurements are plotted in graph611 of FIG. 6 and are similar to those reported for simulations in FIG.5, which show that the shuttle beam displaces 1.6 μm when the actuationvoltage is 70.2V. The discrepancy between predicted and measureddisplacements is due to the microfabrication tolerances and measurementerrors.

Frequency response of the device from the input voltage (with 20-V DCoffset) to the tip displacement is obtained by the LDV. Results showthat the first resonant frequency of the device is located at 10.7 kHz.A second-order model is fitted to the frequency response to characterizethe first mode dynamic response of the device:

$\begin{matrix}{{G_{m}(s)} = {\frac{3{0.5}8}{s^{2} + {655.4s} + {{4.5}52 \times 10^{9}}}.}} & (4)\end{matrix}$

The measured and curve fitted amplitude frequency response are shown ingraphs 612 and 613, respectively; the measured and curve fitted phasefrequency response is shown in graphs 614 and 615, respectively. Thelower resonance obtained empirically is due to microfabricationtolerances and the fact that the simulation did not allow for othersources of damping, such as internal damping of the material.

For mode shape measurements, the surface of the nanopositioner isscanned through the predefined points using the LDV, and the frequencyresponse of the nanopositioner is obtained at each point in order toconstruct the mode shapes. The experimental mode shapes are in goodagreement with the predicted ones reported in FIG. 4. While the firstresonance is measured to be at 10.7 kHz, the higher out-of-planeresonances are beyond 14.1 kHz. The discrepancy in the predicted andmeasured values can be attributed to the microfabrication tolerances.

A commercial UHV STM system (Scienta Omicron UHV VT STM) is used as thetestbed for the experiments. FIG. 13 shows the UHV chamber of suchsystem with base pressure of 4×10⁻¹¹ Torr. As discussed above, the MEMSnanopositioner device 1301 is designed with dimensions compatible withthe tip holder of the STM system. Conductive epoxy is used to affix theMEMS nanopositioner device onto the tip holder and connect the MEMS padsto the three signal pins of the tip holder (i.e. ground, actuation, andtunnel current) with gold wires. Afterwards, the MEMS assembly istransferred into the UHV chamber and mounted on the piezotube 1302, asshown in FIG. 10(b). The sample 1303 used in the experiment isH-passivated Si(100)-2×1 which is mounted upward down above the MEMSassembly. A rapid control prototyping unit (Zyvector, Zyvex Labs)running at 100-kHz sampling frequency is utilized to implementalgorithms required for coarse positioning, establishing the tunnelingcurrent, feedback loop, imaging, and conducting lithography.

In order to prevent tip crash during a scan, a feedback loop is requiredto regulate the MEMS to maintain the tunneling current at a setpoint byrejecting disturbances. Here, the same controller for Z-axis of theoriginal system's piezotube (i.e. proportional-integral) is used for theMEMS. However, in order to remove the quadratic nature of the MEMSelectrostatic actuation, the square root of the control command is usedin the feedback loop.

To construct a topography image, the control command is plotted againstthe XY-scanning pattern. For the STM based lithography, the samplesurface is first imaged, and based on the image, the lattice is detectedand mapped. Then, the STM tip is moved along a predefined trajectorywith respect to the lattice, while the tunneling parameters are set tothose required for lithography.

FIG. 14 illustrates a simplified block diagram detailing a suitablecontrol feedback loop structure. The voltage amplifier, G_(amp)(s),provides the required voltage for driving the MEMS nanopositioner,G_(m)(s), changing the tip-sample gap, δ, which is approximately equalto the barrier thickness:

δ=h ₀ −h _(d) −z,   (5)

where h₀ is the initial distance between the MEMS tip and sample, andh_(d) accounts for the disturbances, e.g. drift. Here δ translates intoa current, as follows:

i=σV _(b) e ^(−1.025√{square root over (φ)}δ).   (6)

In this equation, σ is a constant term that depends on the materialproperties of the tip and sample, whereas V_(b) and φ are the samplebias and barrier height, respectively. The preamplifier (G_(pre)(s))converts the nano-amper level current into a measurable voltage, V_(i).In order to minimize the measurement noise (n), gain (R) of thepreamplifier is typically set to 3 V/nA. The V_(i) signal is discretizedby one of the Analog-to-Digital (A/D) converters available in Zyvectorto be used in the control loop. As pointed out by Eq. 6, tunnelingcurrent is an exponential function of the tip-sample gap, and likewiseof the MEMS motion. Therefore, it is necessary to take the naturallogarithm of the signal to provide a linear dependency, as follows

ln(Ri)=ln(RσV _(b))−1.025 √{square root over (φ)}δ.   (7)

Here, ln(V_(i)) is compared with logarithm of the setpoint, and theresulting error signal is fed into the controller, K(s). Although theerror signal is discrete, the control scheme can be described incontinuous time domain due to the high sampling frequency. Thecontroller here is a proportional integratal controller with thefollowing equation:

$\begin{matrix}{{{K(s)} = {K_{p} + \frac{K_{i}}{s}}},} & (8)\end{matrix}$

based on which the controller command, u(t), is determined. The squareroot of this signal is then taken to eliminate the quadratic dependencyof z displacement on the applied voltage, as pointed out in Eq. 2.Finally, a Digital-to-Analog (D/A) converter in Zyvector converts theresulting discrete signal into an analog one to be used by theamplifier.

FIG. 15 presents a typical atomic-resolution STM image 1501 obtainedwith the MEMS nanopositioner device of the present invention. For thisimage, the tunneling current and bias were set to 0.05 nA and −3 V,respectively. The image resolution was 512×256 pixels, and tip speedduring the scan was 625 nm/s. Various surface features aredistinguishable in this figure, such as step edge 1502, missing Si atoms1503, and dangling bonds 1504. FIG. 15 is an STM image 1510 of thesurface after conducting STM-based lithography with the MEMSnanopositioner device. The pattern defined here is a one-loop spiral.The tunneling current, bias, and dosage were set to 3 nA, 3.75 V, and 3mC/cm for lithography, respectively. As shown in the figure, the MEMSnanopositioner device was able to create a lithography pattern 1511 withtwo-three dimer-row resolution.

The design, fabrication, and characterization of a new 1-DOF MEMSnanopositioner is disclosed that may replace the Z-axis of theconventional STM piezotubes and the STM tip. The device incorporates anintegrated in-plane Si tip suitable for batch fabrication processes. Inorder to enable parallelism, a novel electrical isolation scheme isproposed and implemented to electrically isolate the tunneling signal.This allows current sensors to be put at the tip side. By integratingthe device into the currently available STM systems, the Z-axisbandwidth can be increased beyond 10 kHz, while retaining the same ROM.The functionality of the device was demonstrated by integrating the MEMSnanopositioner device into a commercial UHV STM system and conductingexperiments on a H-passivated Si(100)-2×1 sample in UHV conditions.

The approach of the illustrative embodiments offers a unique opportunityfor parallelism in STM through the design of an integrated STM tip and anovel electrical isolation scheme. This ensures the uniformity of theSTM tips throughout a single wafer.

As used herein, the phrase “a number” means one or more. The phrase “atleast one of”, when used with a list of items, means differentcombinations of one or more of the listed items may be used, and onlyone of each item in the list may be needed. In other words, “at leastone of” means any combination of items and number of items may be usedfrom the list, but not all of the items in the list are required. Theitem may be a particular object, a thing, or a category.

For example, without limitation, “at least one of item A, item B, oritem C” may include item A, item A and item B, or item C. This examplealso may include item A, item B, and item C or item B and item C. Ofcourse, any combinations of these items may be present. In someillustrative examples, “at least one of” may be, for example, withoutlimitation, two of item A; one of item B; and ten of item C; four ofitem B and seven of item C; or other suitable combinations.

In some alternative implementations of an illustrative embodiment, thefunction or functions noted in the blocks may occur out of the ordernoted in the figures. For example, in some cases, two blocks shown insuccession may be performed substantially concurrently, or the blocksmay sometimes be performed in the reverse order, depending upon thefunctionality involved. Also, other blocks may be added in addition tothe illustrated blocks in a flowchart or block diagram.

The description of the different illustrative embodiments has beenpresented for purposes of illustration and description, and is notintended to be exhaustive or limited to the embodiments in the formdisclosed. Many modifications and variations will be apparent to thoseof ordinary skill in the art. Further, different illustrativeembodiments may provide different features as compared to otherdesirable embodiments. The embodiment or embodiments selected are chosenand described in order to best explain the principles of theembodiments, the practical application, and to enable others of ordinaryskill in the art to understand the disclosure for various embodimentswith various modifications as are suited to the particular usecontemplated.

What is claimed is:
 1. A microelectromechanical (MEMS) devicecomprising: a substrate; and a movable structure flexurally connected tothe substrate, capable of moving in relation to the substrate; whereinthe movable structure further comprising two or more segments having atleast one mechanical connection between said segments to providestructural integrity of the moving structure; and wherein the at leastone mechanical connection electrically isolates at least two segments.2. The MEMS device of claim 1 wherein the two or more segments comprise:a first segment having a first electrical routing; a second segmenthaving a second electrical routing; and, wherein the first and secondelectrical routings are electrically isolated from one another.
 3. TheMEMS device of claim 2 wherein: the substrate comprises a singleSilicon-on-Insulator (an) wafer; the first segment and the secondsegment share a device layer of the single SOI wafer; and, the first andsecond electrical routings are included within the device layer.
 4. TheMEMS device of claim 1 wherein the at least one mechanical connection isa bridge made of oxidized silicon.
 5. The MEMS device of claim 1 whereinthe structural integrity allows the MEMS device to withstand a load inthree dimensions.
 6. A MEMS nanopositioner for scanning tunnelingmicroscopy comprising: a substrate having a flat surface defining asubstrate plane, a first electrode, a second electrode and a set ofsubstrate actuator fixtures; a shuttle beam which is movable in aZ-direction, wherein the Z-direction is parallel to the substrate planeand aligned with a longitudinal axis of the shuttle beam, the shuttlebeam further comprising: a tip segment comprising a STM tip electricallyconnected to the first electrode for sensing tunneling current; anactuation segment mechanically connected to the tip segment by a bridgesegment; wherein the bridge segment provides electrical isolationbetween the tip segment and the actuation segment; wherein the actuationand tip segments are flexibly connected to the substrate by a set offlexures; and, wherein the actuation segment further comprises a set ofelectrostatic actuator arms interposed with the set of substrateactuator fixtures wherein the set of actuator arms are electricallyconnected to the second electrode for applying a voltage between the setof actuator arms and the set of substrate actuator fixtures.
 7. The MEMSnanopositioner of claim 6 wherein the tip segment, the actuator segmentand the bridge segment share a device layer of a single SOI wafer. 8.The MEMS nanopositioner of claim 6 wherein the electrical connectionsbetween the STM tip and the first electrode and the electricalconnections between the actuation segment and the second electrode sharea device layer of a single SOI wafer.
 9. The MEMS nanopositioner ofclaim 6 wherein the out-of-plane stiffness of the MEMS nanopositioner ina direction orthogonal to the base plane is higher than the Z-directionstiffness of the MEMS actuator device.
 10. The MEMS nanopositioner ofclaim 6 wherein movement of the MEMS actuator device in a directionorthogonal to the Z-direction and parallel to the base plane isconstrained by the set of flexures.
 11. The MEMS nanopositioner of claim6 constructed as to have a first resonant frequency of 10 kHz or higher.12. The MEMS nanopositioner of claim 6 constructed as to have range ofmotion of about 2 μm (similar to Z-direction motion of conventional STMsystems).
 13. The MEMS nanopositioner of claim 7 wherein the STM tipshares a device layer of the single SOI wafer and is made of silicon.14. The MEMS nanopositioner of claim 12 wherein the STM tip forms at theintersection of three planes: the substrate plane, the Si <111> planeand a plane perpendicular to the substrate plane.
 15. An array of MEMSnanopositioners as in claim 6 sharing a single wafer substrate.
 16. Amethod for fabricating a MEMS nanopositioner having a STM tip in a tipsegment connected to an actuator segment by a bridge segment as in claim7, comprising the steps: a. providing a highly doped SOI wafer with adevice layer, a buried oxide (BOX) layer beneath the device layer and ahandle layer beneath the BOX layer; b. creating the bridge segment inthe device layer; c. forming the other features of the MEMSnanopositioner by etching the device layer; d. forming the STM tip inthe device layer; e. depositing and patterning one or more metal layersto form a set of electrodes including the first and second electrodes onthe MEMS nanopositioner; f. patterning and etching the handle layer backto the BOX layer underneath the MEMS nanopsitioner device; and, g.releasing the MEMS nanopositioner device by etching the BOX layerunderneath a movable portion of the MEMS nanopositioner device.
 17. Themethod of claim 16 wherein the step b of creating the bridge segmentcomprises: b(i). etching the device layer down to the BOX layer todefine the bridge; b(ii). oxidizing the bridge; b(iii) conformallydepositing an oxide layer to reinforce the bridge; and, b(iv). etchingback the oxide layer to expose the device layer except where the bridgewas defined.
 18. The method of claim 17 wherein the step b(i) comprisesdeep reactive ion etching.
 19. The method of claim 17 wherein the stepb(ii) comprises wet thermal oxidation at an elevated temperature. 20.The method of claim 17 wherein the step b(iii) comprises a low pressurechemical vapor deposition (LPCVD) process.
 21. The method of claim 16wherein the step b(iv) comprises reactive ion etching.
 22. The method ofclaim 16 wherein the step c comprises forming the shuttle beam, the tipsegment, the actuator segment including the set of flexures and the setof electrostatic actuator arms, the set of base actuator arms,electrical routings and bonding pads for the electrodes.
 23. The methodof claim 16 wherein the step c comprises deep reactive ion etching. 24.The method of claim 16 wherein the step d further comprises forming thetip body with an angle of about 15 degrees with respect to the Sicrystallographic <110> plane.
 25. The method of claim 16 wherein thestep d comprises deep reactive ion etching concurrently with step c. 26.The method of claim 16 wherein the step d of forming the STM tip in thedevice layer comprises: d(i). depositing a sacrificial oxide layer toconformally cover a portion of the device layer; d(ii). creating anopening in the sacrificial oxide layer, the opening configured to form amask for the STM tip; d(iii). etching the device layer under the openingto form the STM tip; and, d(iv). removing the sacrificial oxide layer.27. The method of claim 26 wherein the step d(i) comprises depositingthe sacrificial oxide layer with an LPCVD process.
 28. The method ofclaim 26 wherein the step d(ii) comprises reactive ion etching.
 29. Themethod of claim 26 wherein the step d(iii) comprises wet etching with aSi etch solution.
 30. The method of claim 29 wherein the Si etchsolution is a KOH or TMAH solution.
 31. The method of claim 16 whereinthe step e(iv) comprises removing the sacrificial oxide layer with oxideetch solution.
 32. The method of claim 31 wherein the oxide etchsolution is a buffered oxide etch solution or concentrated HF.
 33. Themethod of claim 16 wherein the step e comprises one or more processesselected from the group consisting of a sputtering process, a lift-offprocess, an evaporation process, a deposition process, or a dry etchprocess.
 34. The method of claim 16 wherein the step f comprises atleast one of deep reactive ion etching or Si wet etching.
 35. The methodof claim 16 wherein the step g comprises at least one of reactive ionetching or HF etching.
 36. The method of claim 16 wherein the highlydoped SOI wafer consists of device layer a device layer of about 20 μmthickness, BOX layer of about 2-μm thickness, and a handle layer ofabout 400-μm thickness.
 37. A method for fabricating a MEMS device as inclaim 3 comprising the steps: a. forming an oxide bridge between thefirst segment and the second segment as the mechanical connection; b.forming a device geometry within the device layer of the single SOIwafer; c. metallizing a set of electrodes connected to the first andsecond segments; d. removing a portion of the handle layer from the SOIwafer to release the MEMS device.
 38. The method of claim 37 furthercomprising the step of forming an integrated tip structure from thedevice layer of the single SOI wafer in at least one of the two or moresegments.
 39. The method of claim 37 further wherein the formation ofthe integrated tip structure comprises a wet etching process of Si. 40.The MEMS nanopositioner of claim 7 wherein the device layer thickness ofthe SOI wafer is about 19 μm. (The Si device layer thickness at thebeginning of the fabrication process is about 20 μm; during the bridgeoxidization process, around 1 μm-thick Si is consumed everywhere overthe device layer, which reduces the final fabricated device layerthickness to about 19 μm.)
 41. A scanning tunneling microscopecomprising the MEMS nanopositioner of claim 6.